How Not to Get MIL-Stoned

Motor,  Apr 2004  by Bell, Sam

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A careful approach to the diagnosis and repair of problems that trigger the MIL on OBD II vehicles is the best way to avoid the unwelcome weight of comebacks.

The gangster's "cement shoes," long a 20th century Hollywood staple, evoked a dread finality with their promise of an eternal weight from which the wearer would never escape. The bad guys 19th century forebears relied similarly on tying a millstone around their victims necks. These were the massive grinding stones ubiquitous in every grain mill and sandstone quarry in the country, and they were just as effective as the gangster victim footwear.

In recent years, a new incarnation of the old millstone has come to plague many in the auto repair industry. It's a subspecies of comeback that attacks the unwary technician after he extinguishes his customer's Check Engine light. I call it the MIL-stone, because once it's fastened around your neck, you may never be able to free yourself. This article will help you avoid that fate!

The Malfunction indicator Lamp, or MIL, is the current name for the Check Engine light (sometimes also called the Check Money light). This article deals with OBD II-equipped vehicles, which are the most likely to "weigh down" the unwary.

There are three scenarios under which you're likely to have to deal with the MIL. The first is when the MIL's recent appearance prompts your customer's visit. The second is when you observe that the MIL is on during the course of other service you're performing. The third is when your customer has just failed a "plug and play" OBD II-based emissions test.

Where to Start

Statistics indicate that approximately a third of all MIL-related repairs result in a comeback when the MIL returns after having been cleared by a repair facility. I have developed the following protocol to substantially reduce the odds of a MIL-related comeback. At the same time, it increases the efficiency of the diagnosis and repair process.

Perform a thorough customer interview. There are a number of very specific questions you should ask. When did the light first come on? Has it been on steadily or intermittently? Does it ever flash rapidly? When it first appeared, how long had it been since you last started the car, had its oil changed or other service work performed, added gas? Has anyone attempted to fix it? Also, take a moment to explain the realities of OBD II diagnostics. Develop a "script" similar to the one in the box on page 36 that you could hand out to customers. Take a moment to explain the realities of OBD II diagnostics. My favorite analogy is to climbing a ladder: Each step has to be secure before you can proceed to the next one.

Perform a generic scan. Do not use the manufacturers enhanced scan protocols yet.

Read and record all DTCs. Don t erase them now. You'll have to refer to them later.

Record all freeze frame data. Use a form similar to the one shown at the top of page 36 to make certain that no important data is either overlooked or unrecorded. Note any incomplete monitors at this time.

Perform a visual underhood inspection. Check all fluid levels. Also cheek for missing, broken or damaged components.

Formulate a Plan of Attack

Now you're ready to develop an initial plan of attack. The specifics of your plan will vary according to the exact nature of the DTCs present, but it should always include:

*Verifying the exact enabling criteria for the DTCs detected.

*Checking TSBs and performing pinpoint tests to arrive at a correct and complete diagnosis.

*Devising a test sequence to verify that the repair has been effective.

*Clearing and resetting the PCM only after each of the previous steps has been successfully completed.

This is the time to use your enhanced (manufacturer-specific) interface. Be sure to familiarize yourself with available bidirectional control menu selections or functional tests (see advanced modes, next column).

Let's start with a familiar example. The most commonly misdiagnosed code is P0440, which signals an EVAP system malfunction. It's very tempting to assume that this is the result of a loose gas cap. Resist the temptation! The same tests that verify the cap is unsealed will also tell you very quickly if that isn't the problem, so don't move the cap until you know. A loose gas cap is at least as likely to be responsible also for codes P0442, P0455 or P04547. Again, the same warning applies: Test first!

The general rule, in fact, is to test adequately before repairing anything. Have your test equipment handy so you can retest before you return the car. Whenever possible, do all your tests and retests before clearing any codes.

Only once you're confident your repairs are complete should you go ahead and clear the codes. But don't put away your scanner just yet. Use it to be sure your test drive mirrors the freeze frame conditions you recorded earlier. Also, check to see whether the appropriate monitors have run to completion. This focused testing enhances the likelihood of your confirming a complete repair. If, as is likely, some of the monitors are still incomplete, you'll need to decide whether you wish to mark these monitors on your OBD II Freeze Frame and DTC Checklist and risk returning the vehicle now, or keep the vehicle for further testing. Remember, certain onboard test monitors are suspended when specified DTCs are present. Such "blocked monitors" can lead to embarrassing-and costly-comebacks.